New species More than 20 million years ago an Australian koala was leaping possum-like through the trees at night - a far cry from its couch potato cousins that slump in eucalypts today.

The more-agile marsupial was half the size of the modern-day koala and ate younger, softer plants, including possibly fruit.

The descriptions are extrapolated from the discovery of an exceptionally well-preserved partial skull of the newly discovered species found at Riversleigh, in Queensland's far north-west.

Named Litokoala dicksmithi after the Australian adventurer and philanthropist, Dick Smith, the new species is described in the latest edition of Journal of Systematic Palaeontology.

Co-author Dr Karen Black, a palaeontologist at the University of New South Wales , says the fossil remains are rare because it is only the second ancient koala skull to have been found with the snout of the facial region preserved.

"The other species, called Nimiokoala greystanesi , which was also discovered at Riversleigh, had a skull that was very possum-like in appearance," she says.

"Litokoala dicksmithi, however, appears to have been much more closely related to the modern koala with numerous similarities in the skull suggesting a more koala-like, rather than possum-like, face."

Much smaller

Black, and colleagues with Dr Julien Louys and Dr Gilbert Price from the University of Queensland, say the discovery brings the number of known extinct koala species to 18. Today, one species of koala is alive.

"Massive eyes in primates correlates with nocturnality and with increased visual acuity," she says. "Along with the small body size of L. Dicksmithi it suggests it would have been jumping around the tress like possums do today and was really active."

Unlike today's eucalypt-munching koala species, Lito. dicksmithi fed on the rainforest plants that covered much of northern Australia 20 million years ago and may also have eaten some fruit.

Black says koalas did not being to specialise in eucalypt until about 10 million years ago and the fossil record shows koalas still living in rainforests as early as 350,000 years ago.

While the researchers could not be definitive on what caused the demise of the ancient active koala, "because we only have one record" the onset of dryer conditions in Australia about 15 million years ago led to the contraction of rainforest habitats.

This resulted in the loss of many species including perhaps L. Dicksmithi.

Black says while fossils of L. Dicksmithi and N. greystanesi have never been found together they were contemporaneous.

"They would have been living in the same environment," says Black. "[But] Greystanesi has a really complex dentition that suggests it would have been eating more fibrous material such as more-mature plants."

She says there would have been a range of koala species in the Riversleigh region at the time, with each species having its own niche in the environment.